CT Big Bore...Which one do I have??

[707trx250r]

I run a ct 350 pv. The 330 is up to the 77.5mm piston so yours is larger so it is a 350 sized cylinder. It could be a stroker, and be more than a 350,but your piston is a 350 sized piston

[cracer250]

How do I tell if this is a stroker motor or not? I know it has some kind of spacer plate under the cylinder, but is there any way to tell this?

Thanks!

[C-LEIGH RACING]

Heres how it is on the Pro-x cylinders which is same as a CT cylinder.
A 350cc cylinder you can bore in up to a 78.00mm piston, after that size the sleeve edge next to the o ring groove gets too thin to safely use any bigger piston.
It can be done, boring in a bigger piston in the 350 cylinder, but that little thin edge at the o ring groove will break off & kill the engine in short order.
If you can see for sure that it says 80.44 on top of the piston, that means it is a 80.50mm piston realy & that size piston is for a 410cc cylinder kit. What your saying it feels like a CR500 just says it even more that its that size.
The 80.50mm piston is the last bore for the 410cc cylinder & only CT Racing offers the 410cc cylinder kits.
Another thing, by you being able to see numbers eteched into the piston crown, thats telling me its not a flat top piston build which was the way CT built them.
Somebody has replaced the piston with a regular dome piston. More than likely the head squish was cut as well & then again maybe not.
Neil

[cracer250]

That helps a little. I'm still confused I guess....

[rrguy]

I don't know alot about ct kits & its hard to say if someone else rebuilt the motor since the standard kit was installed. The 410 kit is not popular mainly cause it is not reliable as a everyday rider.

The plate could be from the motor having a 87-89 long rod which a some ct kits require a spacer plate for. Or it could be a stroker kit?

Stock displacement is 246cc
66mm bore X 72mm stroke = 246cc
68mm bore X 72mm stroke = 260cc
80mm bore X 72mm Sroke = 370cc
Stroker ?
80mm bore X 82mm stroke+ 410cc
80.5mm bore X82mm Stroke 420cc

[rrguy]

If you find TDC & have a clean rod or something to insert through spark plug hole. You can mark the rod when piston is at very top then reaches very bottom measure that distance between markings & you should have your stroke figured out.

Not much to pulling off head, to measure bore, except I would replace gasket & retorque head bolts.

[D Bergstrom]

Originally posted by rrguy

The plate could be from the motor having a 87-89 long rod which a some ct kits require a spacer plate for. Or it could be a stroker kit?

For CT racing kits, if you have a long rod motor, you must use the spacer plate.

My 330 kit uses a 76mm bore, and the 350 kit I had used a 78mm bore. According to CT, you can over bore 5 times on the 350 kit, but I think that is only a 0.25mm each time, so at the max bore you would be at 79.25mm, so I do not know how you could be at 80.44mm bore with the 350 kit.

I wonder if CT ever made a 370 kit?

76mm bore x 72mm stroke = 326.62cc (330 kit)
78mm bore x 72mm stroke = 344.04cc (350 kit)
80mm bore x 72mm stroke = 361.91cc (maybe a 370 kit?)

You can always try and give CT a call and see what they have to say. I am sure they will be able to help.

Doug

[cracer250]

It's definitely an 80.50mm bore. I need to try and figure out my stroke and I'll post my results. Thanks for the help so far.....

[400EX QUADER]

a 78 bore and a 76 stroke equals 370cc... Not exactly but that's what the 370's are. I believe the 78x77 are what they call a 380?? Kinda a guess. The ct i'm getting is 80.5 bore x 77.5 stroke and that's a 394cc motor...

[C-LEIGH RACING]

The 350 cylinder & a + 5 stroker crank is the 370cc.
What you have figured, the 394cc, is the 410cc cylinder kit CT offers.
Neil